Every style fits a bit differently.
The American Apparel Tri-Blends and 50/50 Blends tend to have a slimmer, athletic fit and don't shrink as much as AA's 100% cotton shirts.
Tultex Blends are a lightweight, super-soft poly-cotton mix, and are advertised as pre-shrunk. However, the ladies' have a lean fit. If in doubt, go one size higher.
Below are links to manufacturer's size charts for more accurate measurements and colors: